I have thought about this as well . . to pay for the middle seat (of 3) for me and my niece. Makes a big difference to have that extra space. I think it's possible but not sure about the PQMs . . but then I don't really care that much. I think you have to check in with BOTH boarding passes so they don't use that empty seat. I sat in the aisle of a 3 seat on Southwest and the large man at the window had his "EXTRA SEAT" boarding pass sitting on the middle seat so I knew. Worked out great for me!